I know this has been discussed before, but my head is spinning with all the different information I've read. To make a long story short, I've got a Sharp DT-100 projector ceiling mounted fed by a ~25ft VGA cable to a computer. The computer is on a different circuit than the projector which is plugged into an outlet I created in the ceiling.

The problem is every so often I get the annoying scrolling ground loop bars on the projector output. I know it's a ground loop because I've run an extension cord from the projector to the plug bar my computer is plugged into. This almost clears it up completely.

From what I've read, I'm tempted to simply put a 3 prong to 2 prong connector on the projector plug and bypass the ground altogether. Now, I've read everything from safety? issues to possible damage to the component. Is there any real truth to this? I mean, I think all the neutral connections in my circuit box are bridged to the ground anyway! As I'm not an electrician, I can't properly understand the theory here. Thanks much in advance for anyone who can clear this up for me.

From what I've read, I'm tempted to simply put a 3 prong to 2 prong connector on the projector plug and bypass the ground altogether. Now, I've read everything from safety? issues to possible damage to the component. Is there any real truth to this? I mean, I think all the neutral connections in my circuit box are bridged to the ground anyway! As I'm not an electrician, I can't properly understand the theory here. Thanks much in advance for anyone who can clear this up for me.

All the neutrals and grounds are bonded at every main panel. Lifting the ground to the projector is not a good idea. You could possibly get a static charge built up in the projector. So what's the solution? I don't know so you'll have to wait for another response. Maybe there is an isolation transformer you can use on the VGA cable,

Do you have spaces open in your service panel? If so, just move the breaker for the circuit that your projector outlet is on to the other leg in the panel so it will be on the same leg as the computer (or vice-versa).

SC

Don't use a 3 prong to 2 prong adapter. That will probably create other problems.

You want all the grounds to go back to the main panel. You don't want several different grounds in the electrical system because in that case there could be different ground levels that could cause electronic devices to malfunction.

Does the projector outlet have a ground that goes all the way back to the electrical panel? If you connected to a water pipe, or its own ground rod then it could be at a different ground potential. If you must use a different ground, then it still should be connected to the single shared ground with a very heavy wire.

In the late 60's/70's many outlets in homes had no ground at all. Apparently, not code now, though. They did have tv's, stereo's and appliances to plug in back then, and they worked.

Soooooo go from there.

The problem is every so often I get the annoying scrolling ground loop bars on the projector output. I know it's a ground loop because I've run an extension cord from the projector to the plug bar my computer is plugged into. This almost clears it up completely.

Tech,

it's interesting how particular data says different things to different people ?

You experiment proves to me the exact opposite that it proves to you.

Seeing as both outlets are on the same earth ground (eventually)- if one was dirty they would both be dirty - and your problem wouldn't change. As noted above - ground loops come from multiple ground paths on the same service - which creates problems due to a difference in potential of the ground.

And yes - eventually they are ALL on the same earth ground - even if you took a special outlet and ran a ground rod for that one particular outlet - because back at the main panel the neutral and grounding buss bars are bonded.

Seeing as plugging into the same circuit almost clears it up - it would suggest to me that what you have is a 60 cycle motor hum - and that the outlets in question are on different legs. One of which is not connected to that particular motor.

I say different legs because - plugged into the same leg as the offending motor - you can get a direct signal via the hot leg- while plugging into the opposite leg could still give you a small amount of interference via the common neutral.

I would try to switch the offending outlet over to the same leg as the remaining equipment.

BTW - what I mean by motor noise could be coming from a refrigerator compressor - a fan motor - an AC compressor - a bath exhaust, a water pump, etc., etc.........

Motors that run some of the time - but not all of the time.

sincerely,

Rod

RG-"I would try to switch the offending outlet over to the same leg as the remaining equipment." definetly isolate the situation, then narrow down what is causing the noise.

Running an additional line from your panel to the projector may be considered if you have an open leg on your panel.

goodluck!

Tech,

it's interesting how particular data says different things to different people ?

You experiment proves to me the exact opposite that it proves to you.

Seeing as both outlets are on the same earth ground (eventually)- if one was dirty they would both be dirty - and your problem wouldn't change. As noted above - ground loops come from multiple ground paths on the same service - which creates problems due to a difference in potential of the ground.

And yes - eventually they are ALL on the same earth ground - even if you took a special outlet and ran a ground rod for that one particular outlet - because back at the main panel the neutral and grounding buss bars are bonded.

Seeing as plugging into the same circuit almost clears it up - it would suggest to me that what you have is a 60 cycle motor hum - and that the outlets in question are on different legs. One of which is not connected to that particular motor.

I say different legs because - plugged into the same leg as the offending motor - you can get a direct signal via the hot leg- while plugging into the opposite leg could still give you a small amount of interference via the common neutral.

I would try to switch the offending outlet over to the same leg as the remaining equipment.

BTW - what I mean by motor noise could be coming from a refrigerator compressor - a fan motor - an AC compressor - a bath exhaust, a water pump, etc., etc.........

Motors that run some of the time - but not all of the time.

sincerely,

Rod


I agree. what you're describing does kind of sound like AC motor noise and removing the Projector from that phase should clear up the problem.